Field of the Invention
The present invention relates to a wireless receiver that receives a radio wave from, for example, a wireless microphone and demodulates an audio signal, in particular, to the wireless receiver which can appropriately display a radio wave reception state on an indicator.
Description of the Related Art
A wireless microphone and a wireless receiver are normally used in one-to-one relation. That is, in multi-wave operation in which a plurality of wireless microphones is simultaneously used on a stage or the like, the same number of wireless receivers tuned with transmission frequencies of the respective wireless microphones are used.
In this case, according to Japanese Radio Law, for example, as a frequency band that can be used for a type B wireless microphone, 30 channels are allocated for a specified low power radio device, in which a range of 806.125 to 809.750 MHz is equally divided in units of 0.125 MHz.
By the way, a reception state of the radio wave received in each wireless receiver can be known by displaying a reception strength (also referred to as a reception level) on an indicator or can be known from the number of pieces of lighting of the indicator. What is displayed on the indicator is preferably in proportion to the received radio wave strength ideally.
However, there has been a problem that, in a case where each wireless receiver has a plurality of antennas such as in a diversity system, there is a variation in a detection level (detection voltage) of a receiving circuit (detection element) of each of the antennas, and as a result a difference occurs in the display of the number of lighting segments on the indicator despite the same electric field strength.
For example, in a case where characteristics of receiving circuits of two antennas, Ant_A, Ant_B, are different from each other, as illustrated in a graph (a detection voltage relative to electric field strength) of FIG. 5, the detection voltages relative to the electric field strengths E1, E2, E3, E4, E5 have different values from each other as illustrated in FIG. 6 (on the graph of FIG. 6, ∘ (a white circle) indicates a detection voltage of the antenna Ant_A, Δ (a white triangle) indicates a detection voltage of the antenna Ant_B).
Here, the number of lighting segments of the indicator (1 to 5) corresponding to the detection voltage (a numerical value in the table is an example) is set in advance as illustrated in a lookup table of FIG. 7. Accordingly, when the detection voltages are different from each other although the electric field strengths are the same between the antenna Ant_A and the antenna Ant_B, there is a possibility that a difference occurs in the number of lighting segments of the indicator as illustrated in FIG. 8.
To address the problem that received electric field strengths are different between the antennas, Japanese Unexamined Patent Application Publication No. 08-223097 A discloses a received electric field strength detection circuit that adjusts a variable resistor of a resistive potential divider circuit so that a received electric field detection signal RSSI (Received Signal Strength Indication) output from a receiving circuit 1 and a received electric field detection signal (RSSI) output from a receiving circuit 2 have the same voltage, when electric fields of the same level are inputted to a first antenna and a second antenna.
However, in a configuration of an invention described in the patent application No. 08-223097 A, a variation in detection characteristics is eliminated by adjusting the detection voltage to be same at a predetermined point (adjustment point) of the electric field strength, and there has been a problem that the variation of the received electric field strength becomes greater as the electric field strength is far from the adjustment point.
That is, there has been a problem that, when the electric field strength away from the adjustment point is received, the difference occurs in the number of lighting segments displayed on each indicator corresponding to each of the antennas.